A carbonylation process of a methanol method (an acetic acid process of a methanol method) is known as an industrial method for producing acetic acid. In this process, for example, methanol and carbon monoxide are reacted in the presence of a catalyst in a reaction vessel to produce acetic acid. The reaction mixture is evaporated in an evaporator, and the vapor phase is purified in a lower boiling point component removal column and subsequently in a dehydration column so that product acetic acid is prepared. Alternatively, product acetic acid is prepared via a higher boiling point component removal column subsequent to the dehydration column, and further, a product column.
In such a process for producing acetic acid, organic iodine compounds, such as hexyl iodide, are produced as by-products in the reaction system, and are included as a very small amount of impurities in the product acetic acid. When acetic acid containing organic iodine compounds is used as a raw material for producing vinyl acetate, the palladium catalyst is degraded, and hence there is a need to reduce the organic iodine compound concentration in the acetic acid to an order of a few ppb. Therefore, conventionally, the organic iodine compound concentration in the acetic acid has been reduced to as low as possible by using a cation exchange resin substituted with silver ions. However, in an adsorptive removal method of organic iodine compounds that uses such a silver-substituted ion exchange resin (hereinafter, sometimes referred to as an “IER”), there is a problem that corroded metals (also referred to as “corrosion metals”) derived from corrosion in an apparatus, such as iron, nickel, chromium, and molybdenum, and the like present in the process stream undergo ion exchange with the silver in the ion exchange resin, whereby useful silver dissolves into the acetic acid and flows out of the system, causing the organic iodine compound removal life of the ion exchange resin to decrease. Further, as a consequence of that, there is also the problem that the concentration of corrosion metals etc. and the silver concentration in the product acetic acid increase, causing the quality of the product acetic acid to deteriorate.
Patent Literature 1 discloses, a process in which, in order to suppress deterioration in the life of an ion exchange resin due to corroded metals, acetic acid is purified using an ion exchange resin composition including a metal-activated ion exchange resin containing a specific amount of metal-functionalized sites and a non-metal-functionalized ion exchange resin containing non-metal-functionalized sites. Further, Patent Literature 2 discloses a method in which, in order to suppress corrosion of a dehydration column, an alkali component for neutralizing hydrogen iodide, which is a factor in apparatus corrosion, is added and mixed into the dehydration column or the dehydration column feeding solution.
However, none of the above-described methods are capable of sufficiently suppressing deterioration in the life of the ion exchange resin due to corroded metals.